If you use Cree XP-G LEDs, 3 per tank, spaced 3 inches apart in a row across the middle of the tank, raised 6 inches above the top of the tank, with 2 inches of substrate in the tank, with 500 mAmps of current per LED, and with 60 degree optics: You should get about 65 micromols of PAR at the substrate and about 150 micromols at the water surface. That would be medium to high light, probably more light than you will want to use. So, you can use the dimming control to reduce that lower. With only 3 LEDs you will get lots of shimmer if the water surface is at all rippled.
At only 500 mAmps of current, you don't need a great heatsink or any cooling fan. I would use a piece of 3 inch by 1 inch by 1/8 inch thick extruded aluminum channel, like what you can find on Ebay, one seller there is Online Metal Supply, but there are other internet stores that sell aluminum extrusions. You can mount the LEDs on the inside surface of the channel, and the "legs" then act to shade your eyes from the LEDs. If you like the look of polished aluminum you can make this look pretty good too. Of course you would mount the drivers somewhere other than on the heatsink.